A magnetic recording and reproducing apparatus comprises means to transport a magnetic tape. There has been used reel drive means for transporting the magnetic tape at constant velocity by directly controlling rotation of reels without any capstan and pressure roller.
In such reel drive means, there is a variable winding diameter with which the magnetic tape is wound on a winding reel and a variable winding diameter with which the magnetic tape is supplied from a supply reel as the magnetic tape is transported. Thus, rotational speeds of the reels are required to be controlled in accordance with the winding diameter of the magnetic tape thereon.
A conventional method of controlling the rotational speeds of the reels will be described with reference to FIG. 6.
A magnetic tape is transported from a supply reel 22 to a winding reel 21. Supposing that a winding radius of the winding reel 21, when the magnetic tape begins to be transported, is Ro, a desired transport velocity of the magnetic tape is V, and the time lapsing after the magnetic tape begins to be transported is t, a rotational angular velocity .omega..sub.1 of the winding reel 21 after time t lapses is determined by the following expression; ##EQU1## Accordingly, if the rotation of the reels is so controlled that the actually detected rotational angular velocity .omega.r.sub.1 of the winding reel 21 is coincident with the rotational angular velocity .omega..sub.1 operationally determined by the aforementioned expression, then the actual tape transport velocity Vr can be the desired value of velocity V.
However, since the conventional method controls the rotation of the reels based on an operational form of time t from the time when the tape begins to be transported, it is very difficult to manage the time t. For example, once a difference between the the rotational speed .omega..sub.1 by the operation and the actual rotational speed .omega.r.sub.1 occurs, it is accumulated, which disadvantageously appears at the rotational speed .omega..sub.1 by the operation.